Shadow at the Water's edge.

Lore has built up over the ages about creatures that live in the ocean, creatures of startling beauty, with the upper body of women, and the tails of fish.
But what if these creatures were different from what we thought they were? What would happen when one of them needs help? Seeking out the aid of a teenage boy, who is more involved in their world than he knows.

When Alex wakes up in the hospital, ranting about a girl and water, he is assured that his hallucinations were a side effect to almost drowning, and he was unconscious for nearly two days. He is released from the hospital, and life seems like it should return to normal...but he's still haunted by his "hallucinations" He still knows what he saw was real.
He saw a shadow at the water's edge.